In general, when a multilayer structure containing a gas-barrier resin layer such as a saponified ethylene-vinyl ester-based copolymer (hereinafter sometimes referred to as EVOH) or a polyamide-based resin is subjected to the retorting (a hot-water treatment), it is known that gas-barrier performance is lowered by penetration of water into the gas-barrier resin layer from an edge or the like of the multilayer structure to destroy intermolecular hydrogen bonds.
Heretofore, against the problem, there has been proposed a technology for suppressing the lowering of the gas-barrier performance by using a resin composition containing a hydrate-forming salt such as monosodium phosphate, sodium pyrophosphate, sodium sulfate, or silica gel blended into an EVOH layer as a desiccant, wherein the desiccant absorbs the water penetrated into the resin composition after the hot-water treatment as crystal water to prevent the destruction of the intermolecular hydrogen bonds (for example, see Patent Document 1).
However, according to the method, the viscosity of the resin composition increases with time in a melt-kneading step at forming and the increased viscosity causes deposition of the resin in an extruder. Also, the viscosity of the deposited resin further increases and thus the resin is still more difficult to be discharged, so that there is a problem that a thermally degraded product of the resin is generated and handling properties of the resin get worse. Moreover, there is a room for improving the gas-barrier properties after the hot-water treatment.